1. Field of the Invention
This invention relates to improvements in a trouble diagnosis device for diagnosing as to whether an exhaust gas recirculation system is normally operating or not in accordance with a pressure difference between an intake pressure during execution of exhaust gas recirculation and an intake pressure during stopping of exhaust gas recirculation.
2. Description of the Prior Art
Most automotive vehicles are equipped with an EGR (exhaust gas recirculation) system in which a part of exhaust gas of an engine is fed back through an EGR passage to an intake air passageway to accomplish an emission control of NOx from the engine. If trouble arises in the EGR system, it is a matter of course that the emission control cannot be normally carried out. Additionally, such trouble is usually difficult to be noticed by a driver, and therefore there is the possibility that the vehicle is driven for a long period of time with a troubled condition of the EGR system.
In view of the above, a trouble diagnosis device for the EGR system has been proposed as disclosed, for example, in Japanese Patent Provisional Publication No. 62-51746. With this trouble diagnosis device, first a deviation in engine speed per a predetermined time and a deviation in throttle valve opening degree per a predetermined time are measured in an EGR operating range in which the exhaust gas recirculation is carried out. Subsequently, a judgement is made as to whether each deviation is below a predetermined value or not. In case of being below the predetermined value, the engine operation is judged to be within a steady state operating condition. In this steady state operating condition, an EGR control valve is temporarily closed to stop the exhaust gas recirculation. Thus, intake pressures Pon, Poff are detected respectively when the EGR control valve is opened and closed. If a pressure difference .DELTA.P (=Pon-Poff) is out of a predetermined range, it is judged that a malfunction exists in the exhaust gas recirculation system.
However, difficulties have been encountered in the above discussed trouble diagnosis device, in which there is the possibility of sharply lowering a frequency or chances of carrying out the trouble diagnosis by the above method in which the judgement is first made as to whether the engine operation is within the steady state operating condition or not upon directly detecting the fluctuation in engine speed or throttle valve opening degree. In order to achieve the trouble diagnosis, it is also necessary that the engine operation is within the EGR operating range. More specifically, for example, in a case that the throttle valve opening degree has instantaneously changed and returned to an original value, a judgement is made such that the engine operation is not within the steady state operating condition if the throttle valve opening degree change exceeds a predetermined value, though the intake pressure in an intake air passageway hardly changes under the effect of response retardation in pressure in an air intake system. In such a case, trouble diagnosis for the EGR system cannot be carried out, thereby unnecessarily lowering the frequency of the trouble diagnosis.
Further difficulties may be encountered in the above discussed trouble diagnosis device, in which it is difficult to make a precise judgement upon merely comparing the pressure difference .DELTA.P with the predetermined range. More specifically, if the intake pressure Pon in the intake air passageway has changed owing to the variation in throttle valve opening degree even when the exhaust gas recirculation is normally carried out, it has been known that the pressure difference .DELTA.P also changes with the intake pressure change. Accordingly, in case that the above-mentioned predetermined range is a fixed range defined by fixed values, there is the possibility of judging as an occurrence of trouble in the EGR system a response to any special engine operating condition even if the exhaust gas recirculation is being normally carried out. Additionally, the pressure difference .DELTA.P becomes smaller than that in the condition where the exhaust gas recirculation is normally carried out, in a case when the EGR control valve is not normally operated or in a case when the EGR passage is clogged. In contrast, the pressure difference .DELTA.P becomes larger in a case that an orifice disposed in the EGR passage is accidentally taken off. In such a case, it is impossible to detect the occurrence of trouble in the EGR system. Furthermore, if an exhaust gas recirculation rate is out of a predetermined range, it cannot be possible to make a such a precise trouble diagnosis for judging an occurrence of trouble.